Radiation-type thickness gauges have been in use for many years and are generally employed in locations or situations, such as hot metal rolling mills, where contact-type thickness gauges cannot be used.
Radiation-type thickness gauges generally comprise a source of penetrating electromagnetic radiation (such as X-rays or gamma rays), a scintillation crystal (such as NaI) sensitive to this radiation, and a photomultiplier or photodiode for detecting the light output of the scintillation crystal and converting it into an output voltage which is substantially proportional to the intensity of radiation incident on the crystal.
Radiation intensity is defined as the photon flux, i.e. the number of photons emitted per second by the radiation source. Individual photons have energies ranging from approximately 5-150 KeV (for X-rays) to 60-1000 KeV (for gamma rays).
Once the radiation intensity incident on the crystal is calibrated, such as by placing a standard metal sample of known alloy composition and thickness between the radiation source and the photomultiplier, the output of the photomultiplier can be correlated with the thickness of an unknown material placed between the radiation source and photomultiplier. Preferably, the photomultiplier output is digitized and the correlation of the thickness measurement is accomplished through the use of a programmable data processor. Such measurement and correlation schemes are shown, for example, in U.S. Pat. Nos. 4,009,376 and 4,119,846.
One drawback to the operation of radiation-type gauges which use scintillation crystals is that the response of such crystals is not strictly linear, especially when there are large changes (a factor of approximately 100 or greater) in the intensity of radiation incident on the crystal. If the crystal has been operating under a high level of incident radiation (for example 10.sup.10 photons/sec. for an X-ray source or 10.sup.6 photons/sec. for a gamma ray source), and this incident radiation is suddenly reduced to a lower level (for example 10.sup.7 photons/sec. for an X-ray source or 10.sup.4 photons/sec. for a gamma ray source), the light output of the crystal does not immediately fall off. Instead, there is a slow decay in the light output of the crystal due to residual phosphorescence (so-called "afterglow") over several seconds, or even minutes, so that accurate measurements cannot be made by the gauging system until the crystal "afterglow" has disappeared.
A related problem is so-called crystal "hysteresis" which is a reversible change in the output of a crystal during irradiation, i.e. the non-linear response of the crystal to radiation will be different depending on whether the radiation intensity is increasing or decreasing. Both afterglow and hysteresis contribute to the non-linear response of a crystal under conditions of rapidly changing incident radiation. Furthermore, every crystal has its own unique decay characteristics, so that measurements on a "standard" crystal cannot be readily used for calibration purposes.
Several proposals have been made in an attempt to overcome the problems associated with crystal afterglow or hysteresis. For example, in U.S. Pat. No. 4,044,261 the effects of phosphorescent afterglow in a scintillation detector are reduced using a filter network in the detecting circuit. In U.S. Pat. No. 4,245,157 hysteresis and afterglow response of a scintillation crystal is reduced by irradiating the crystal with high energy electrons while heating the crystal to irreversibly change the response characteristic of the crystal material. In U.S. Pat. No. 4,272,677 the afterglow response characteristic of a crystal due to low energy electron bombardment is used to define a "drift-stable" peak in the low-energy spectrum of the crystal. In U.S. Pat. No. 4,079,257 auxiliary radiation sources are used to calibrate the photomultipliers of a scintillation camera. In U.S. Pat. No. 4,223,388 radiation from a reference source is measured and used to form a "correction table" which is applied to measured values of samples to correct for non-linearity of the scintillation camera components due to uneven characteristics of photomultiplier tubes and ageing of circuitry. Finally, U.S. Pat. Nos. 3,732,420 and 3,769,508 describe a calibration system for a scintillation camera which scans an area of interest and detects the area of maximum activity (a so-called "hot-spot"). The intensity value of the "hot-spot" is averaged over time and used to set the value of the maximum density area to be recorded on a chart or film.
While various attempts have been made to reduce the effects of afterglow or hysteresis in a gauging system, none of the described techniques specifically corrects for the non-linear response of a scintillation crystal due to such afterglow or hysteresis.
It is therefore a primary object of the present invention to provide a technique for calibrating a scintillation crystal and, more particularly, to correct for the non-linear response of such a crystal due to afterglow and hysteresis during subsequent gauging cycles.